As is well known, conventional electrical plugs are held in electrical contact with conventional electrical receptacles by frictional contact between the contact members of the plug and receptacle. This arrangement is quite acceptable for most applications in which the appliance is not moved during use. Such applications include for example lamps, radios, and other such appliances. However, there are applications, such as in the use of power tools and the like, in which electrical appliances are moved during use and as often happens excessive tension applied to the electrical cord of the appliance disconnects the plug from the receptacle. Not only is this inconvenient, it can create a hazardous condition. There are other situations in which it is essential that the plug not become inadvertently disconnected from the receptacle. The frictional contact forces generated by conventional plugs and receptacles is insufficient to provide the security required in the latter situations.
There are also situations in which it is desirable to have a visible indication of electrical continuity in a conductor and, particularly, through a plug-receptacle connection.